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President Trump’s decision to withdraw from the Paris Agreement on climate change has raised questions about the effectiveness of the accord, and how that will change without the US.

In his announcement, Trump incorrectly claimed the deal would avoid just 0.2C of warming. In fact, nine separate studies show, on average, that full implementation of current climate pledges would avoid 1C of warming, compared to a business-as-usual world.

Analysis by Carbon Brief finds that if the US reneges on its Paris pledge and takes no action to reduce emissions, it could result in around 0.2C to 0.3C additional warming, whereas a delay in implementation of four or eight years would have minimal impact.

Carbon Brief explains how these temperature estimates are made and explores the impacts of Paris, with and without US participation.

The first round of NDCs run through 2025 or 2030 and are not sufficient to meet the overall aim of Paris of keeping warming well below 2C above pre-industrial temperatures. As part of the agreement, countries agreed to reconvene every five years to assess progress and ratchet up their efforts.

Nevertheless, the cumulative impact of the NDCs – which reflect both current climate policies and new commitments – is significant. At least nine separate groups have released white papers or published peer-reviewed research since mid-2015 assessing the impact of the Paris Agreement on future warming. A summary of their projections for a business-as-usual world without Paris and a world where countries meet current Paris commitments is shown in the figure below.

On average, these groups estimate that implementation of current commitments in the Paris Agreement would result in around 1.1C less warming in 2100, with estimates ranging from 0.17C to 1.6C. The groups differ a bit both in estimates of future reductions and of how much warming there would be in a business-as-usual scenario without any climate action. Some include just the NDCs submitted as part of the first round of the Paris Agreement, while others include further commitments made by some countries after Paris, for example additional pledges made at the COP22 UN climate summit in Marrakech last November.

The main challenge of determining the climate impact of the Paris Agreement is how to estimate post-2030 emissions, since the NDCs only extend through 2025 or 2030. Different groups take different approaches, but generally assume that Paris commitments remain in place post-2030 and that emissions continue on a similar path.

The Climate Interactive model, shown below, assumes that countries with absolute emissions reduction targets simply maintain constant emissions after 2030. For example, China’s emissions would peak in 2030 and stay level through 2100. Countries such as India, with commitments to reduce emissions as a percent of GDP, are assumed to resume business as usual emissions rates of change after 2030.

The figure below shows the carbon emissions trajectory in a business-as-usual case, a Paris commitment case, and an emissions trajectory that avoids more than 2C warming.

Projected CO2 emissions under business-as-usual, Paris commitments and 2C trajectories. The 2017-2030 period covered by existing commitments is highlighted, while emission projections after 2030 are shown by dashed lines. Business as usual and Paris trajectories were provided by Climate Interactive from their C-ROADS model; the 2C trajectory is based the IPCC’s RCP2.6 scenario. Chart by Carbon Brief using Highcharts.

Bjørn Lomborg’s 2015 paper in the journal Global Policy is the only study that finds a number close to the 0.2C “tiny, tiny amount” mentioned by Trump. Lomborg assumes similar emission reductions to most other studies for the core Paris period of 2016-2030. Rather than have long-term emissions stay around 2030 levels, however, Lomborg assumes that countries continue on a near business-as-usual emission trajectory after 2030. In his model post-2030 emissions are only reduced each year by roughly the amount of reductions in 2030.

For example, Lomborg assumes that China’s CO2 emissions will continue to rise through 2080 even under the Paris Agreement, despite its pledge to peak emissions by 2030 and growing evidence that emissions may have already peaked.

Other studies, such as those in 2015 by Allen Fawcett and colleagues in Science and in 2016 by Joeri Rogelj and colleagues in Nature find Paris reductions of around 1.2C, similar to most other groups examined.

Impact of US emissions

While dramatic, the decision by the US to withdraw from the Paris Agreement does not necessarily commit the world to any additional warming. Prior to announcing the withdrawal, which will not actually take effect until November 2020, the Trump administration was already moving to gut climate policies put in place by the prior administration. A world where the US remained in the agreement, but failed to put any effort into meeting its targets, would have likely had the same impact on domestic emissions.

What really matters is what the US does in the future: does it completely eschew mitigation for the next 83 years and continue emitting CO2 at its current or even an increasing rate? Or does a future administration bring the US back into the fold and pursue deep reductions in CO2 emissions?

To help answer this question Carbon Brief used a simple climate model to analyse different emission scenarios to try to determine what the Trump administration’s policies will mean for global temperatures at the end of the century and our ability to limit warming to well below 2C in 2100.

Carbon Brief looked at five different scenarios for future US emissions. The first is a world where the US begins an immediate pathway toward deep decarbonisation, meeting its Paris commitments, continuing on to reduce emissions by 80% in 2050, and reaching net-zero emissions by the end of the century.

The second is a variation of this deep reduction scenario where the US abandons climate policies until a new administration in 2021 reverses course and pursues deep reductions. The third is a similar scenario where a new administration in 2025 (rather than 2021) does the same.

The fourth looks at a low business-as-usual scenario where emissions slowly return to where they were back in 2005 and remain roughly constant, and the fifth examines a high business-as-usual scenario where rapid economic and population growth swamp continued reductions in emission intensity, resulting in increased emissions through 2100.

Future CO2 emission scenarios for the US. The “Business-as-Usual High” scenario and “Deep Reduction” scenario were provided by Climate Interactive. The Business-as-Usual Low scenario was based on projections from the IPCC AR5 Scenario Database AMPERE 3 Base GCAM model. Delay scenarios are adapted from the Deep Reduction scenario assuming that the Deep Reduction mitigation trajectory begins at the later date, and that the maximum rate of reductions achieved is maintained until convergence with Deep Reduction. Chart by Carbon Brief using Highcharts.

These scenarios are purely illustrative; models that attempt to predict the future are always wrong but sometimes useful, and these provide a range of possible paths that the US could take.

It is hard to foresee a world where climate policy and mitigation are completely ignored for the rest of the century, and there is a strong argument that existing trends in energy systems and cost reductions in renewables and storage should at least keep emissions flat, if not lead to future declines. Action on a state or local level within the US could also drive additional declines in emissions regardless of policies adopted at the federal level.

To estimate the impact of these scenarios on global temperatures, Carbon Brief assumed the rest of the world pursues policies aimed at limiting warming to 2C following the IPCC’s RCP2.6 scenario. For each scenario, Carbon Brief used a simple climate model to determine the increase in temperatures over time from additional US carbon emissions relative to “deep reduction”.

Estimated additional contribution to global mean surface temperatures due to US emissions relative to the Deep Reductions scenario, assuming that the rest of the world follows a RCP2.6 pathway. Calculated by Carbon Brief using SimMod. Chart by Carbon Brief using Highcharts.

Here, the business-as-usual world where the US continues to increase its emissions results in just under 0.3C additional warming, similar to projections widely reported in the media. A somewhat more plausible scenario where US emissions remain flat through the end of the century yields just under 0.2C additional warming.

If the US delays action for eight years before pursuing deep reductions, there would be around 0.02C additional warming, while delaying only four years results in less than 0.01C additional warming.

While 0.2 to 0.3C additional warming may not sound like much, it would make it nearly impossible for the world to limit warming to 2C. Similarly, a small amount of warming can make a big difference at the margin, as Carbon Brief previously explained when discussing the difference between 1.5C and 2C warming scenarios.

A temporary delay in US emission reduction of a few years would have a much smaller climate impact, assuming that the rest of the world does not follow in the US’s footsteps.

Conclusion

While it is possible to meet a 2C target with temporary delays in US mitigation, the large additional emission reductions needed compared to current Paris commitments are daunting.

The most severe potential impact of a delay in US action on climate change is not that US emissions will be higher for a period of time, but rather that other countries may be less likely to meet their commitments or pursue the deeper reductions needed to set us on a 2C trajectory with the US not participating in the process.

Some businesses are already arguing that the US withdrawal puts them at a competitive disadvantage, and these voices will become louder as countries move closer to the net-zero-emission future required to limit warming to 2C.

Note: The “deep reduction” US emission scenario provided by Climate Interactive is consistent with a world where there is a 50% chance of keeping warming in 2100 below 2C. There are other more aggressive scenarios that focus on a 66% chance that involve bringing emissions to zero more quickly as well as the large-scale deployment of negative emissions technologies like biomass with carbon capture and storage (BECCS).
This article was updated on June 20th 2017 to clarify the approach taken by Bjorn Lomborg's study.